In the practice of liquid chromatography and other analytical methods involving flowing liquid streams, a sample injector is often used. A sample injector receives a liquid sample from a syringe, and when switched, transfers the sample at high pressure into a chromatographic column. One successful type of injector includes a largely disc or cylinder-shaped rotor with a passage extending therethrough that receives a syringe needle. The rotor has a face extending perpendicular to its axis of rotation, which lies facewise adjacent to the face of the stator. The stator has passages therethrough which connect to an external sample loop. When the syringe needle is inserted into the rotor and operated, the sample fills (fully or partially, depending on the technique chosen) the volume of the sample loop and of the stator passages connected to the loop. When the rotor is turned, solvent from a high pressure pump pumps the sample through the loop and corresponding stator passages into the chromatographic column. U.S. Pat. Nos. 4,182,184 and 4,506,558, both by Bakalyar, describe injectors of this type, that are devoid of any passage between the syringe needle and loop that would waste part of the sample.
If the sample is larger than the volume of the external sample loop (plus the small additional volume of the two stator passages and one rotor groove), then the volume of sample injected into the column is precisely controlled by the size of the sample loop. Different size sample loops can be used, where different sizes of samples are to be injected. However, for very small volumes that are to be injected, the volume occupied by the passages plus the smallest length of sample loop which can connect the two passages, may be too great. It would be desirable if a sample injector were available which could store and inject a predetermined precise volume which was very small. It would further be desirable if such an injector could be readily changed between a configuration for injecting a very small volume, and configurations for injecting one or more much larger volumes.